In open and narrative the same is only true of a detachment your army can include units of any factions.

An interesting wording on an astra millitarum stratagem take cover.

When an enemy selects a friendly unit as a target in the shooting phase. That unit gets +1 to saving throws until the end of the phase.

It specifically doesn't state astra millitarum you can use it on any unit, so take a small detachment for the ability to unlock AM stratagems and troll with a 3++ om your Stormsurge or get your biggest troll face out for a 2++ Riptide with nova reactor

If you want something nasty to think of, FTGG activates on any charging target, not just friendly. So in a free 4 all you're enemy could be charging something else and you can join in for free shooting! Probably won't happen a lot, but I think that's pretty cool!

It is a pretty strange thing to be doing - if done for game advantage.

On the other hand I quite fancy getting my Kroot Korsairs (Kroot based Eldar Corsair conversions) out for a little action and I am aware that taking the Void Dreamer as a Farseer (the nearest equivalent in 8th) opens up some combo nastiness. Putting Doom on a target unit then hitting it with massed Rail Rifles could get nasty very fast. I am really not sure if that is a bit too cheesy for me - much as I want to put those conversion on the table.